The present invention relates to a control device for a hydraulic slide valve. It can be applied notably to the positional servo-control of hydraulic slide valves with one or more stages for the transmission of mechanical control power whose application requires very short response times and very high stability as is the case in certain braking systems for example.
There are many approaches to the transmission of mechanical control energy. These include systems based on pneumatic, electrical or hydraulic devices. The pneumatic devices have very long response times for many applications and the electrical devices require large quantities of electrical energy that are incompatible with the constraints of on-board systems. These two types of devices are furthermore relatively costly. The control systems of modern vehicles, notably those applied to braking or to automatic guidance for example, require increasingly short response times corresponding, for example, to passbands of over 10 Hz. Furthermore, the reduction of the total cost of the vehicles or of the equipment requires an ever increasing reduction in the cost of their constituent elements, hence also in the cost of the above-mentioned control elements. At present, it is only hydraulic devices that are likely to be best suited to the transmission of mechanical control power on board vehicles or mobile equipment at low cost and with sufficient speed as required by new technical and economic conditions.
The devices generally used in the field of hydraulic controls implement complete devices such as servo-mechanisms based on delivery valves or pressure valves for example. The hydraulic devices have characteristics and types of architecture that constitute a homogeneous whole. Their flow-rate control function or pressure control function is constituted by a set of several stages consisting of slide valves and springs.
As a rule, these servo-mechanisms have two stages. The first stage is a linear actuator. It controls an arm provided with a shaft or bar that is in motion by magnetization. This magnetization results from the action of a current flowing through a solenoid valve associated with the shaft. The second stage is a hydraulic amplifier consisting of a slide valve and a draw-back spring for example. The mechanical inertia values, the resonance values and the time constants complicate the servo-control and restrict the speed of movement of the devices. It is nevertheless possible to improve the performance characteristics of these systems, notably those pertaining to their response time or passband. However, this is obtained at the cost of an increase in their complexity that considerably raises their costs of manufacture. These raised costs may possibly be compatible with aeronautical applications but are incompatible with applications to earth-based vehicles, for example mass-produced vehicles with the requirement of low cost.
There is a known approach that is capable of reducing the above-mentioned drawbacks, notably in simplifying the servo-control by making the phenomena of resonance and instability less dominant. This approach consists in replacing the above actuator, which is of the linear type, by a rotary actuator transmitting a translational motion to the hydraulic slide valve, for example by means of a link-rod, and replacing the springs in the second stage by a drawback spring acting on the rotary actuator. While the latter approach, which is less complicated and more economical, clearly improves the performance characteristics notably by eliminating the resonance phenomena, it has hitherto never been possible, in the tests performed, to attain the desired response times, which are of the order of 0.1 second for example, corresponding to passbands of the order of about 10 Hz.
One drawback of these hydraulic systems, whether they are linear or rotary, is that they may show instability.
A known approach to eliminate this instability, namely the oscillations of the hydraulic slide valve, consists in adding a spring with a high draw-back force to this slide valve. This approach however is not suited to systems that require short response times, in particular because of the draw-back force of the spring.
The aim of the invention is to overcome the above-mentioned drawbacks, notably by enabling the making of hydraulic transmission systems capable of complying with instructions as regards position with short response times while at the same time remaining stable.